1. Field of Invention
The present invention relates generally to a semiconductor process. More particularly, the present invention relates to a method of fabricating a self-aligned shallow trench isolation (SASTI).
2. Description of the Related Art
A capacitor is a part of a dynamic random access memory (DRAM) that stores data. In order to decrease data error and memory refresh, the capacity of the capacitor has to be increased to improve the efficiency of the operation. One method of increasing the capacity of the capacitor is to increase the surface area of the capacitor, wherein the steps of this method comprise: forming a deep trench (DT) in a substrate, and forming a DT capacitor in the deep trench. Because the depth of the deep trench is very deep, a surface area of the capacitor is increased.
However, the DT capacitor utilizes the surface area of the substrate, so a shallow trench isolation (STI) is fabricated on the top of the DT capacitor in order to utilize the surface area of the substrate effectively. A conventional method of fabricating a STI is described as below:
Referring to FIG. 1A, a p-type substrate 100 is provided first and a pad oxide layer 102 and a mask layer 104 of silicon nitride are formed sequentially on the substrate. Two deep trenches 110 are formed in the p-type of substrate 100. An n-type doped region 120 is formed in the p-type substrate 100. An internal electrode 125 of the bottom capacitor and an internal electrode 128 of the top capacitor are formed in the deep trenches 110. A buried plate 121 and an n-type doped region 120 are formed in the p-type substrate 100 located at the lower part of the deep trenches 110. The n-type doped region is very thin and is connected to an n-type doped region 120. The buried plate 121 serves as an external electrode for the DT capacitor. A dielectric layer 123 with a very thin thickness is formed in between the internal electrode 125 of the capacitor and the buried plate 121. The internal electrode 128 of the top capacitor, the p-type substrate 100 and the n-type doped region 120 have a thick collar 127 for isolation. A top surface of the internal electrode 128 of the top capacitor is lower than the surface of the p-type substrate.
Referring to FIG. 1A, an n-type doped polysilicon layer 130 is filled into the deep trench 110 and it covers the top part of the internal electrode 128 of the top capacitor. The top surface of the doped polysilicon 130 is lower than the surface of the p-type substrate 100. A thermal process is carried out to allow doping ions of the n-type doped polysilicon layer 130 to distribute into the surrounding p-type substrate 100. A doped region 140 is thus formed.
Referring to FIG. 1B and FIG. 1A, a photoresist layer 156, which has an opening 158, is formed on the p-type substrate 100. The photoresist layer 156 exposes the mask layer 104 located between the two deep trenches 110 and a portion of doped polysilicon layer 130. The photoresist layer 156 is utilized as a mask to etch away all the parts located in the opening 158. A trench 160 is formed (shown in FIG. 1B). A remained doped polysilicon layer 130 and the doped region 140 are utilized as buried S/D 150.
Referring to FIG. 1C, the photoresist layer 156 is removed followed by filling the trench 160 with silicon oxide to form a STI 170. The mask layer 104 and the pad oxide layer 102c are removed, and a gate 179 and S/D regions 180 located at both sides are formed on the p-type substrate 100. The S/D regions 180 are electrically connected to the internal electrode of the top capacitor 128 by the buried S/D 150.
FIG. 1B illustrates a conventional method of defining the trench 160 that is not a self-aligned method. An alignment problem between the internal electrode of the top capacitor 128 and the trench 160 often will occur. The quality of the capacitor will be affected. From FIG. 1C, most of the doped polysilicon layer 130 is removed during the formation of the trench 160. A contact area between the buried S/D 150 and the internal electrode of the top capacitor 128 is very small; thus, the resistance in that region becomes very large and affects the efficiency of the device.
The present invention provides a method of fabricating a self-aligned shallow trench isolation, suitable for a device of a DT capacitor. A preferred embodiment of the present invention provides a method of solving the alignment problem between an internal electrode of a capacitor and a STI, and the high resistance problem between a S/D region and the internal electrode of the capacitor.
The method of the present invention comprises the following steps. A mask layer, two deep trenches and two internal electrodes of a capacitor are sequentially formed on a substrate. Two conductive layers are used to completely fill the two deep trenches. A portion of the mask layer is removed to expose the conductive layers. Then, two spacers are formed on the exposed sides of the two conductive layers, and two doped regions are formed in a substrate located next to the two conductive layers. A patterned photoresist layer is formed to expose at least the spacers located in between the two deep trenches and mask layer. The photoresist layer and the spacers are utilized as masks to etch away the exposed mask layer and expose a portion of the substrate. The photoresist layer is utilized again as a mask to etch the exposed spacers and a portion of the exposed substrate. Sequentially, a remained portion of the photoresist layer and a portion of the conductive layers are removed. Simultaneously, a remained mask layer is used as a mask to remove a portion of the exposed substrate, and a trench is thus formed. The remained portion of the conductive layers completely cover the two internal electrodes of the capacitor. The remained portion of the conductive layer and the doped regions are utilized as a buried S/D. Finally, a shallow trench isolation is formed in the trench.
The present invention further provides a stack structure for a DT capacitor and a STI. The structure comprises: a substrate comprising two deep trenches, two internal electrodes of a capacitor, two conductive layers and two doped regions and a STI. The two internal electrodes of the capacitor are located in the two deep trenches. The two conductive layers are located on a top of the two deep trenches, and completely cover a top portion of the two internal electrodes of the capacitor. Top surfaces of the conductive layers are lower than a surface of a substrate located outside the two deep trenches, and a surface of a substrate in between the deep trenches is lower than the top surfaces of the conductive layers. A trench is thus formed. The substrate, which is located in between two deep trenches has a side portion and a central portion, and the side portion is higher than the central portion. The two doped regions are next to the substrate of the two conductive layers. The two doped regions and the two conductive layers are utilized as two buried S/D. A doping type of the two doped regions and the two conductive layers is different. The STI is filled into the trench.